STARCH FORMATION IIQ 



the chloroplast. The mass of starch formed in the 

 leaf, and the dependence of this process upon illumina- 

 tion, can be strikingly demonstrated to the naked eye 

 by covering part of the leaf during the day with tinfoil 

 while still on the plant and leaving the rest exposed. 

 If in the afternoon the leaf is cut off, killed with boiling 

 water, the colour taken out with alcohol, and the leaf 

 is then placed in a dish and flooded with iodine solution, 

 the part of the leaf that was exposed to light will 

 quickly turn blue-black owing to the staining of the 

 starch grains as the iodine gradually penetrates the 

 tissues, while that part of the leaf protected from 

 light remains colourless. 



The function of the chloroplasts as starch formers 

 is quite distinct from their function as sugar producers. 

 It does not depend directly on the photosynthesis of 

 sugars or on the green colour of the chloroplasts, but 

 on the actual concentration of sugar in the leaf. The 

 leaves of many plants do not form starch at all, and 

 the concentration of sugar in the leaf necessary to lead 

 to starch formation varies considerably in different 

 species. The power of forming starch from sugar is 

 shared by the colourless plastids (leucoplasts) found 

 in the colourless cells of the plant, particularly in seeds, 

 tubers, etc., but also in stems and roots, where large 

 quantities of starch are formed from the sugar which 

 comes to these cells from the leaves. Chloroplasts 

 themselves can form starch from sugar which they 

 have not made, as can be proved by the experiment 

 of floating detached living leaves on sugar solution in 

 the dark, when starch will appear in the chloroplasts. 

 As the illumination decreases the rate of sugar 

 forihation likewise decreases, and when it falls below a 

 certain intensity — ^in natural illumination about sunset — 



